The phosphorylation status of NONPHOTOTROPIC HYPOCOTYL3 affects phot2-dependent phototropism in Arabidopsis

The blue light photoreceptors, phototropin 1 (phot1) and phot2, and their signal transducer, NONPHOTOTROPIC HYPOCOTYL3 (NPH3), are activators of the phototropic responses of Arabidopsis hypocotyls. In a recent study, we reported that the control of NPH3 phosphorylation at serine 7 (S7: or S5), S213, S223, S237, S467, S474 (or S476), and S722 (or S723) contributes to the photosensory adaptation of phot1 signaling during the phototropic response. Phosphomimetic NPH3^SE mutant and unphosphorylatable NPH3^SA mutant on those serine residues function efficiently under blue light conditions at fluence rates of 10^-5 µmol m^-2 s^-1 and 10^-3 µmol m^-2 s^-1 or more, respectively. We here demonstrate that phosphomimetic NPH3^SE, but not unphosphorylatable NPH3^SA, promotes phot2-dependent phototropism under blue light condition at 100 µmol m^-2 s^-1. This result suggests that phot1 negatively controls phot2 signaling through the dephosphorylation of NPH3 at those residues and that the hyperactivation of phot1- and phot2-NPH3 complexes does not occur at the same time under high intensity blue light. We hypothesize that the dephosphorylation of NPH3 on those serine residues suppresses both phot1 and phot2 signaling, which results in different impacts on phot1- and phot2-dependent hypocotyl phototropism due to the differences in the photosensitivity and activation levels of phot1 and phot2.